DVB is the European consortium standard for the broadcast transmission of digital terrestrial television. DVB systems transmit a compressed digital audio/video stream, using multi-carrier modulation, such as orthogonal frequency division multiplexing (OFDM). Another popular method of transmitting signals is digital video broadcasting-terrestrial (DVB-T). When broadcasters employ DVB-T, the transmitted signals do not travel via cable. Instead, they move via aerial antennas to a home based receiver.
DVB-T broadcasters transmit data with a compressed digital audio-video stream using a process based on a Moving Picture Expert Group (MPEG)-2 standard. These transmissions can include all kinds of digital broadcasting, including high definition television (HDTV). MPEG-2 signals represent an improvement over the older analog signals, which require separate streams of transmission.
By way of background, in multi-carrier systems, such as OFDM systems, serially-inputted symbol streams are divided into unit blocks. The symbol streams of each unit block are converted into N number of parallel symbols. After the conversion, these symbols, which include data, are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to an Inverse Fast Fourier Transform (IFFT) technique, and are transmitted via the channel in time domain.
In addition to data, these OFDM symbols also include scattered pilot carriers (SPC), continuous pilot carriers (CPC), and reserve tone pilot carriers. These pilot carriers (signals) are used for frame synchronization, frequency synchronization, time synchronization, channel estimation, transmission mode identification, and/or phase noise tracing. The data and the pilot carriers constitute the useful part of the OFDM symbol. As understood by those of skill in the art, these OFDM symbols also include a guard interval.
Once the OFDM symbols are captured on a receiver side of the OFDM system, they must be demodulated. OFDM demodulation procedures include, for example, a Fast Fourier Transform (FFT) step, an equalizing and de-interleaving step, and a synchronizing step, among others.
Synchronization of OFDM receivers is performed to locate the useful part of each symbol to which the FFT is to be applied. This synchronization, generally performed in the time domain, can be characterized as coarse synchronization (e.g., initially performed during an acquisition period) and fine synchronization. Fine synchronization improves upon the results achieved during coarse synchronization enough to provide reliable demodulation. Time domain correlation is used to achieve initial symbol timing synchronization. This process may also be used to detect the guard ratio of the OFDM transmission. However, there could be an ambiguity in closely spaced guard ratios worked out using this method.
In DTV receivers there can be significant timing and frequency offsets at the input to the demodulator due to the tuning step sizes and due to tolerances of the reference frequency sources used in low cost consumer applications. These errors are operating temperature and ageing dependent. Good DTV receiver designs are expected to estimate and correct for relatively large offsets without any external help and without any impact on acquisition time.
In DVB-T2 initial frequency offset estimation is carried out using the P1 preamble OFDM symbol. However, the P1 symbol is 1K OFDM and hence the accuracy of this frequency estimate is inadequate for 16K and 32K OFDM. Frequency estimation may be improved using continuous pilots. However, DVB-T2 has eight possible pilot patterns. The pilot pattern that is used is signaled, but to get at this signaling data it is necessary to do LDPC decoding. LDPC decoding will not work if there is a frequency offset because the decoded OFDM carriers will be wrong. Furthermore, there could be ambiguities in the guard interval. The guard interval is initially estimated using time domain correlation, but under noisy conditions it is not practical to differentiate between closely spaced guard ratios like 1/16 and 19/256.
The algorithm proposed does frequency offset estimation, pilot pattern detection, spectral inversion resolution, accurate guard ratio estimation and timing error estimation for DVB-T2 without the need to extract any signaling information via LDPC decoding. Hence the demodulation synchronization algorithm can be implemented in the programmable processor before activating the LDPC hardware engine. This simplifies the implementation and reduces the acquisition time. This algorithm is also applicable to DTV receivers like ISDB-T and DVB-T that do not have preamble symbols or LDPC decoders. Given the above problem of the conventional technique, improvements are needed.